1. Field of the Invention
This invention relates to a process for regenerating catalysts employed for the conversion of hydrocarbons. More particularly, the invention relates to the regeneration of fluid cracking catalysts which have become poisoned by various metal contaminants.
2. Description of the Prior Art
Various processes such as cracking, hydrocracking, etc., are known for the conversion of hydrocarbons to lower molecular weight products. The catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to desirable fuel products such as heating oils and high-octane gasoline. Illustrative of "fluid" catalytic conversion processes is the fluid catalytic cracking process wherein suitably preheated high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely-divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated riser reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons.
In the catalytic process, some non-volatile carbonaceous material, or "coke," is deposited on the catalyst particles. As coke builds up on the catalyst, the activity of the catalyst for cracking and the selectivity of the catalyst for producing desirable products diminish. The catalyst particles may recover a major proportion of their original activity by removal of most of the coke by a suitable regeneration process. Catalyst regeneration is accomplished by burning the coke deposits from the catalyst surface with an oxygen-containing gas, such as air. Many regeneration techniques are practiced commercially whereby a significant restoration of catalyst activity is achieved. The burning of coke deposits from the catalysts requires a large volume of oxygen or air and produces substantial quantities of CO and CO.sub.2. Ordinarily, the regeneration is conducted at a temperature ranging from about 1050.degree. to about 1250.degree. F. The effect of any increase in temperature is reflected in an increased rate of combustion of carbon and a more complete removal of carbon, or coke, from the catalyst particles.
A major problem often encountered in the practice of fluid catalyst regeneration is the phenomenon known as "afterburning," which is descriptive of the further combustion of CO to CO.sub.2. The operators of fluid catalyst regenerators avoid afterburning because it could lead to very high temperatures which are damaging to equipment and possibly to the catalyst particles.
More recently, as operators have sought to raise regenerator temperatures for various reasons, elaborate arrangements have also been developed for control of regenerator temperatures at the point of incipient afterburning by suitable means for control of the oxygen supplied to the regenerator. However, with the control of afterburning, the flue gas from catalyst regenerators usually contains very little oxygen and a substantial quantity of CO and CO.sub.2. In order to substantially eliminate the CO from the flue gas and to recover heat energy from the combustion of CO to CO.sub.2, the regenerator flue gas is generally sent to a CO boiler wherein the combustion of CO is performed.
Most recently, there has appeared in the literature, e.g., U.S. Pat. Nos. 3,838,036 and 3,844,973, various techniques for substantially eliminating both afterburning and the presence of CO in the regenerator effluent flue gas. These techniques generally involve the use of relatively high regeneration temperatures, e.g., 1275.degree.-1400.degree. F., and the presence of relatively high concentrations of O.sub.2 in the regenerator so that there is substantially complete combustion of the catalyst coke and the resultant CO to CO.sub.2 in the fluidized dense phase and the dilute phase catalyst zones of the regeneration vessel.
Unfortunately, the techniques recently developed for the regeneration of cracking catalysts to substantially eliminate the presence of CO in the regenerator effluent flue gas are not readily adaptable to the regeneration of fluidized catalysts which have been employed for the conversion of heavy petroleum fractions such as deasphalted oils and residua. It is found that the cracking of such heavy petroleum fractions results in the deposition of relatively high amounts of coke and metal poisons upon the spent catalysts The substantially complete combustion of the enormous amounts of coke deposited upon the catalysts for the conversion of heavy petroleum fractions would require enormous amounts of air or oxygen to completely convert the carbon and resultant CO to CO.sub.2. In addition, the presence of such large quantities of coke upon the spent catalyst will produce substantial quantities of CO in the regeneration zone which will be very difficult to completely burn in the regeneration zone without resulting in "afterburning."
Accordingly, in the regeneration of the severely coked catalyst particles, it will be desirable to control the amount of oxygen or air delivered to the regenerator so that there will be insufficient amounts therein to completely burn the resultant CO to CO.sub.2. The CO present in the regenerator flue gas will then be conventionally removed in a CO boiler. Such a regeneration procedure is, however, deficient in that the regenerated catalyst is still partially poisoned by the presence of metal contaminants from the feedstocks such as nickel and vanadium.